Hello my name is***** look forward to assisting and am very sorry to hear that you are having this problem.
I want to warn you that it is possible you will need to replace a part in order to fix this.
There are a few different things that can cause this and a couple different parts so we would need to troubleshoot to find out for sure.
This would mean you need a new part then, just in case lets check anyway.
While holding the power button gently push the lens to the side.
Test this with each of the 4 sides, if it is off center to one side doing this should get it to retract while holding the power button.
If you hear a click while doing it do not worry that is normal.
If the same thing happens I really hate to have to tell you this but from what you are describing I am sorry to say you indeed are going to require a new part. I know this is not what you want to here but I want to make sure I am honest with the information I provide you. You would need to replace the lens zoom unit in order to fix this issue. I wish there was another way but with everything you have posted I am confident this is the problem with your camera. Over time these parts deteriorate and need to be replaced.
A new zoom unit normally costs between £35 and £50. It would take about 30 minutes to an hour for a tech to replace it at their shop rate. Most Shops charge between £50 and £75 per hour.
Due to the price some people do decide to replace it instead especially since you can get the same model used cheaper that repair cost here: Click Here
If you were to do that I would keep the old one so you have spare parts if anything else ever fails. If you have any questions or need any help following these steps click on reply to expert and let me know. If not please consider leaving a positive rating for my help it is greatly appreciated. Please note you are rating me only not your Canon or this site.